The present application is directed to hip replacement procedures and, more particularly, to methods and devices for preparing and sizing the femur to receive a femoral component.
Hip replacement procedures involve the replacement of the hip joint formed by the head of the femur and the acetabulum of the pelvic bone. Hip replacement procedures include the preparation of the femur for receipt of a first component and preparation of the acetabulum to receive a second component. The two components engage together to replace the hip joint. Numerous surgical approaches exist exposing the joint to perform hip replacement surgeries.
Initially, the transtrochanteric approach was most popular, as advocated by the father of hip replacement surgery, John Charnely. This involved exposing the boney prominence to the side of the human hip, that is attached to the femur and to which the abductor muscles attach, to stabilize the hip/pelvis during gait. Over time, surgeons began going either in front of (anterior orientated approaches) or behind (posterior based approaches) the trochanter.
Anterior orientated approaches had many variations and names (Anteriolateral, Watson-Jones, direct lateral, Smith-Peterson, direct anterior) many of which involved detaching the abductor muscle without removing the trochanter. These approaches all shared a low postoperative hip dislocation rate. However, the most popular of these approaches anterior the trochanter had the drawback of longer recovery and potentially weakened abductor muscle. Posterior orientated approaches were less destructive to the abductor muscle, but with the disadvantage of a higher postoperative dislocation rate.
The ideal approach would have the advantages of approaches anterior to the trochanter in regard to dislocation, without the disadvantages of increased muscle damage/recovery/weakness.
The damage to the soft tissue and muscles with anterior based approaches occurs during the preparation of the femur for the femoral component. The use of cement to obtain implant fixation has largely been replaced by cementless (bone ingrowth) implant designs, that require the intra-medulary femoral canal to be reamed and/or rasped to an exact silhouette of the final prosthetic implant. Reamer-based systems have the advantage of accurately machining bone of varying density, similar to the accuracy of a drilling a pilot hole for a screw. The disadvantage of reamers is that occasionally more bone/soft tissue gets reamed than intended. These elongated reamers include cutting edges that extend along their length. The cutting edges extend outward beyond the canal and may contact against the nearby tissue during rotation of the reamer. This contact damages the tissue thus causing a more invasive procedure with additional recovery time for the patient.
As muscle sparing approaches (anterior to the trochanter) have developed, shorter stems that do not require bone preparation using full length straight reamers have been introduced. Techniques that require specific untested implant designs are at risk for unforeseen complications. For rasp prepared short stem implants the risk is for prosthetic subsidence and/or periprosthetic fracture.
A technical consideration, shared by all surgical approaches, involves the sizing and placement of both prosthetic components. In regard to the femoral component, correct size directly affects the depth the component can be seated into the bone. Placed too deep and the leg ends up being left short, but more commonly, placing the component proud, leads to lengthening of the leg. Another drawback of posterior based approaches is the need to place the component proud to obtain adequate hip stability.
While success has been obtained with many different femoral component designs, the most successful and popular implant designs involve the use of rigid tapered reamer preparation. Reamer design initially involved reamers with parallel surface, but now are more popularly designed with a slight taper (3 degree) that is an identical match to the taper design in the femoral implant. This creates an ice cream cone shape to the reamed femur to which an ice cream shape implant is inserted.
To summarize, the potential problems with straight (full length) reaming systems include the following:                1. Difficulty in correctly sizing the canal for receipt of the femoral component. Progressively larger reamers are inserted in the canal as part of the sizing process. Resistance to reaming is a major determinant of when to stop increasing the diameter of the reamer (and subsequently the femoral component size). When using full length reamers, often it isn't possible to determine if the resistance is occurring at the tip, middle or top of the reamer.        2. When the reamers are tapered, the surgeon needs to simultaneously determine not only when the reamer engages cortical bone (correctly sized), but also to what depth the reamer is inserted.        3. Incorrectly reaming too deep requires correction by increasing the size of the femoral component, sometimes to a size larger than desirable.        4. Incorrectly reaming too shallow, if uncorrected, can over-lengthen an extremity.        
While the majority of cases are correctly reamed, based on preoperatively templating and surgical experience, the outliers resulting in repeating surgical steps, resulting in either error, prolonged surgical time and/or increased surgical trauma. It is for this reason, surgeons transitioning to muscle sparing approaches anterior to the trochanter, have circumvented the disadvantages of full length reamers, by using shorter rasp based systems. As these implants evolve to shorter length, new challenges and complications are potentially introduced. Rasp based systems have unique sizing challenges and have been associated with higher periprosthetic fracture rates, both intra and postoperatively. Additionally, shorter stems have potentially less stability and surface area for bone attachment and ingrowth, and subsequently painful implants requiring revision.
Therefore, there is a need for a hip replacement procedure that allows surgeons using long stem, reamer based systems to continue transition to muscle sparing approaches, based anterior to the trochanter, without having to experiment with new shorter stem designs. The technique should include methods and devices for sizing the femur, depth of placement and receipt of the femoral component without increased damage to nearby tissue and abductor muscle.